The loss, which was Barcelona's first in the league this season, means that Atletico Madrid are now level on points with the Catalan outfit at the top of the La Liga table.
However, the Argentine defender is confident that his side will overcome their recent struggles in the coming weeks.
"It hurts to be beaten. Nobody likes to lose but let's not forget we are still leaders," Mascherano told the club's official website.
"We have to win the games that are left this year, beginning with the matches against Cartagena and Villarreal."
Barcelona face Cartagena in the Copa del Rey on Friday.